1. Technical Field
This invention relates to the art of using exhaust gas oxygen (EGO) sensors for detecting catalyst failure, such catalysts being of the type that converts automotive engine emissions to non-noxious gases and water vapor.
2. Discussion of the Prior Art
There is growing concern that to improve air quality in the United States, emission related components, such as a catalyst, must be monitored on board the vehicle to determine any malfunction. Catalyst monitoring has been and still is the least understood, both conceptually and practically, of the emission related components.
EGO sensors have been used in the past, in pairs, to monitor catalysts, one sensor being placed upstream from a catalyst and the other placed downstream of the catalyst, and the signals from each of such sensors were evaluated to determine any difference that would indicate the catalyst was degraded. It is presumed by such prior art that a properly operating catalyst would be capable of dampening the periodic rich to lean excursions resulting from the limit cycle A/F feedback control or intentionally generated in the exhaust stream and that a substantial loss in catalyst performance through loss in actual conversion activity and/or oxygen storage activity would result in a decrease in this dampening ability of the catalyst. This generalized approach to catalyst monitoring compares complex signal features from both devices, each of which is disposed in a different environment and exposed to different exhaust gas locations, and furthermore presumes that there is a correlation between catalyst oxygen storage, sensed signal features, and catalyst performance. Often there is no such correlation. However, since each sensor uses a similar construction, including a catalytic coating that acts as a microcatalyst, failure based on the inability of the main catalyst to convert emissions may be hidden or masked by the sensor itself.
Patented variations of the two sensor catalyst monitoring system have utilized or compared many sensor signal characteristics, including voltage amplitude, phase shift, and frequency ratioing. In some cases, an artificial change in the sensor signal is created by modulation of the engine A/F ratio which, it is hoped, will more clearly show the onset of catalyst degradation. Unfortunately, all of such prior art approaches have at least the following characteristics in common: they expose the electrodes of the sensors to different emission gases, the sensors inherently have construction variations in tolerances and aging, and a decision as to catalyst degradation cannot be made without comparison to an artificial reference. Such prior art sensor system approaches are inaccurate not only due to such sensor differences but also are not able to sense a difference in oxygen between equilibrated and nonequilibrated oxygen conversion or combustion.
What is needed is a system that more reliably monitors catalyst degradation or inadequate engine combustion.